1. Field of the Invention
The present invention relates generally to an optical disk device, and more particularly to the reproduction a wobble signal in an optical disk device used with a recordable or rewritable optical disk such as a CD-R (compact disk recordable) or a CD-RW (compact disk rewritable).
2. Description of the Related Art
Conventionally, in a recordable or rewritable optical disk such as a CD-R, CD-RW, DVD-R, DVD-RW, or DVD-RAM (digital versatile disk random-access memory), guide tracks are “wobbled” so as to record ATIP (absolute time in pregroove) or ADIP (address in pregroove) on the pregroove. The ATIP or the ADIP (hereinafter referred simply to as “address information” is used to acquire the current position of the optical disk, and according to this address information, a control of the recording or reproduction is performed.
The address information can be reproduced by taking out wobble components contained in an electric signal that can be obtained by receiving the light reflected from the optical disk. For example, in the case where a photodetector comprised of four quadrant photodiodes (having light receiving surfaces A, B, C and D) is used under the condition that the light receiving surfaces combination A, D and the light receiving surfaces combination B, C are divided in a radial direction of the optical disk, the signals that can be obtained with respect to the two divided light receiving surfaces combinations are in phase when the reflected light, EFM-modulated (eight to fourteen modulation) depending on the presence of a pit, is received, whereas wobble signals with respect to the two divided light receiving surfaces combinations are 180° out of phase. Accordingly, by determining the difference between the output signals of the respective two divided light receiving surfaces, it is possible to remove EFM-modulated components, thereby taking out a wobble signal alone.
FIG. 6 shows in block diagram a wobble signal reproducing system incorporated in a conventional optical disk device. Two photodetectors A and B divided in the radial direction of an optical disk receive the light beams reflected from the optical disk and output electric signals corresponding to intensities of the reflected light beams (in the case of quadrant photodetectors, it may be considered that two light receiving surfaces together form a photodetector A, and remaining two light receiving surfaces together form the other photodetector B). The output signal of the photodetector B is supplied to a sample-hold circuit (S/H) 50, and the output signal of the photodetector A is supplied to a sample-hold circuit (S/H) 52. The sample-hold circuits 50, 52 are supplied with a sampling pulse, which is synchronized with the timing of a light beam having a reproduction power. Based on the sampling pulse, the sample-hold circuits 50, 52 perform sampling of the output signals during a period of the light beam with reproduction power.
The sampling during the period of the light beam having reproduction power is achieved for a recording operation in which a light beam with a recording power and a light beam with a reproduction power are alternately irradiated on to the optical disk. In a playback operation where information recorded on the optical disk is read out, the light beam irradiated on the optical disk always has a reproduction power. Accordingly, the sample-hold circuits 50, 52 can be passed over.
The signals that have been sampled and held by the sample-hold circuits 50, 52 are supplied to amplifiers 54, 56 where they are amplified at a predetermined amplification factor or gain. The outputs from the respective amplifiers 54, 56 are then supplied to a differentiator 58, which determines the difference between the two signals to remove EFM-modulated components. The output signal of the differentiator 58 is supplied to a band-pass filter (BPF) 60. The BPF 60 extracts from the input signal only components of frequencies near a predetermined wobble frequency (22 kHz, for example) and outputs the extracted components as a wobble signal. In the case where the address information is modulated by a frequency-shift keying (FSK) method in which two FSK frequencies are 21 kHz and 23 kHz with a center frequency being 22 kHz, the BPF 60 is designed to have a narrow pass band (with a bandwidth of ±1 kHz) at the center frequency (filter center frequency) of 22 kHz. By thus designing the BPF 60, it is possible to extract signals with frequencies in a range of 21 kHz to 23 kHz.
As the demand for a high-speed recording operation has increased in recent years, an improvement has been proposed wherein for driving an optical disk, a constant angular velocity (CAV) control is used in place of a constant linear velocity (CLV) control in order to improve the transfer rate in the vicinity of an outer peripheral portion of the optical disk and to lower the load on a spindle motor. In this case, since the wobble signal recorded on the track is set to have a constant frequency (22 kHz) when driven under the CLV control, the driving of the optical disk under CAV control causes the center frequency of the wobble signal to vary in conjunction with the radial position of the optical disk. This makes it difficult for the BPF 60 having a narrow pass band to extract the wobble signal over the entire area of the optical disk.
It may be considered that a wobble signal to be recorded on the optical disk is adapted for the CAV control. There arises another problem that the resulting optical disk cannot maintain the compatibility with the conventional CLV control.